1. Field of the Invention
The present invention relates generally to thermoelectric batteries which convert thermal energy directly into electrical energy by use of a continuous concentration electrochemical cell. More specifically, the present invention relates to an improved thermoelectric battery based on the generation of an electric current utilizing a hydrogen ion concentration gradient.
2. Description of the Background Art
U.S. Pat. No. 3,231,426, issued Jan. 25, 1966, discloses a continuous concentration cell in which a voltage is obtained and an electric current is generated between a cathode immersed in concentrated sulfuric acid and an anode immersed in dilute sulfuric acid. The reaction cycle which is set up between the electrodes is: ##STR1##
During operation of the cell, the concentrated sulfuric acid solution is diluted by water generated at the cathode, while the dilute sulfuric acid solution becomes more concentrated due to the generation of acid at the anode. The difference in acid concentration between the two solutions must be maintained in order to provide continuous generation of electrical energy. The system disclosed in U.S. Pat. No. 3,231,426 maintains the acid concentration gradient by heating the concentrated acid solution to distill off water generated at the cathode. The water which is continuously distilled from the concentrated acid solution is cycled to the dilute acid solution to continually provide dilution of the acid which is generated at the anode. The above-described system accomplishes its intended purpose. However, during operation of the cell, large amounts of water must be continually distilled from the concentrated acid solution, recondensed and then cycled to the dilute acid solution. This distillation process is not energy efficient and requires additional equipment to handle the copious amounts of water which must be distilled and circulated during operation of the system.
The system described in patent application Ser. No. 788,999, assigned to the present assignee, improves upon the system disclosed in U.S. Pat. No. 3,231,426 by providing a system in which a sodium sulfate buffer is utilized to generate electric energy without requiring the energy inefficient distillation and circulation of relatively large amounts of water. In this improved system, a buffered solution containing sodium sulfate and sodium bisulfate is substituted for the dilute acid anode solution in the previous sulfuric acid concentration cell. Use of this buffered solution instead of dilute acid allows the generation of electric energy without requiring distillation and recirculation of large amounts of water.
During operation of this improved system, sodium bisulfate is generated at the anode and sodium sulfate is consumed. In addition, the sodium bisulfate is thermally converted to sodium sulfate, water, and sulfur trioxide. The sodium sulfate and water are recycled to the anode solution to replenish sodium sulfate which is consumed. The sulfur trioxide is recycled to the concentrated sulfuric acid (cathode) solution where it combines with water generated or collected at the cathode to form sulfuric acid. This continual thermal conversion of sodium bisulfate provides continual replacement of the sodium sulfate and sulfuric acid consumed during operation of the system.
While useful for its intended purpose, the above-described buffered sulfuric acid system requires a temperature of 450.degree. C. in order to thermally regenerate the electrochemical cell reactants. This relatively high temperature makes such a system unsuitable for low temperature uses where the highest temperature available is about 250.degree. C. or less, such as in an energy-efficient system which can use waste heat from an external system as the heat input for the above-described thermal regeneration process. A particular application of such a system is for the generation of electricity from the waste heat from an internal combustion engine. It is projected that the electric power requirements for automobiles or trucks will increase by as much as a factor of ten, from 500 watts presently to 5 kilowatts. The use of three 50 volt alternators to supply this power would reduce the fuel economy by 30 to 50 percent and would decrease the acceleration of the vehicle. Consequently, a need exists in the automotive industry for a system which can produce electrical energy from the waste heat of an internal combustion engine, at high efficiency and high power density by direct conversion of heat to electricity.
Another area where only relatively low temperatures (below about 250.degree. C.) are available for thermal regeneration of reactants in batteries similar to those previously described is in undersea applications, where subterranean heat sources are within the range of 80.degree. to 150.degree. C. (176.degree. to 302.degree. F). A need exists for the development of an undersea power source which can be used, for example, in undersea oil recovery to control valves in oil wells located on the sea floor to permit the delivery of oil from the sea floor to the surface. Presently, these valves are controlled from ground equipment by cables approximately 8 to 20 miles long, which extend from land to the sea floor and consist of electrical conductors and hydraulic fluid conductors. However, these cables frequently are damaged by subsea landslides or fishnets, and the oil delivery system must be periodically shut off in order to repair or replace the damaged cables. Consequently, substantial savings could be realized if these cables and associated ground equipment could be replaced by an undersea power source to provide power to a microprocessor which could control the undersea oil well valves. Thus, a need exists for an undersea power source which has heretofor been unavailable.
Further need exists in industrial environments where the ability to use low grade waste heat from industrial processes to generate electricity would significantly reduce cost.
The present invention is directed to meeting the need for a power converter to generate electrical energy from thermal energy at a relatively low temperature and at high efficiency and high power density.